An ultrasonic transducer carries out at least one of the conversion from an electrical signal to ultrasonic and the conversion from ultrasonic to an electrical signal, and is used as a probe for medical imaging and nondestructive testing.
A form of ultrasonic transducer is a capacitive electromechanical conversion device.
U.S. Pat. No. 6,958,255 describes a technology relating to such a capacitive electromechanical conversion device, and FIG. 11 is a sectional view of the basic structure thereof. A silicon single-crystal layer 1101 has electrical conductivity and an electrically insulating layer 1106 is formed on the surface thereof. On the electrically insulating layer 1106, a depressed portion 1104 is formed. To the surface on which the depressed portion 1104 is formed, a membrane member 1102 is bonded in an approximate vacuum. The depressed portion 1104 is an empty space sealed to maintain the approximate vacuum, constituting a cavity. Here, in the present Conventional Example, the depressed portion and the cavity are the same space, so sometimes both are shown with the same reference number 1104.
The present Conventional Example is an example where the silicon single-crystal layer 1101 forms a substrate for a capacitive electromechanical conversion device and also functions as an electrode. The membrane member 1102 is supported by a support portion 1103 formed in the electrically insulating layer 1106. An electrode 1105 is formed on the membrane member 1102 in the center of the cavity 1104, and a capacitor is formed between silicon single-crystal layer 1101 and the electrode 1105.
FIGS. 12A to 12D illustrate the main steps of the process for producing a capacitive electromechanical conversion device, shown in FIG. 11. First, a substrate 1107 is formed in the preceding step illustrated in FIG. 12A. On the substrate 1107, the silicon single-crystal layer 1101, the support portion 1103, the depressed portion 1104, and the electrically insulating layer 1106 are formed. In addition, a silicon-on-insulator (SOI) wafer 1108 is prepared. The SOI wafer 1108 has a structure where a handle layer 1109 comprising a silicon single crystal, a buried oxide film layer 1110 comprising silicon oxide, and a device layer 1111 comprising a silicon single crystal are laminated together in this order. The device layer 1111 is to be the membrane member 1102 in a subsequent step. In addition, the handle layer 1109 and the buried oxide film layer 1110 function as membrane support layers until the device layer 1111, i.e., the membrane member 1102, is bonded to the substrate 1107.
As illustrated in FIG. 12B, the surface on which the support portion 1103 for the substrate 1107 is formed and the device layer 1111 of the SOI wafer 1108 are directly bonded together. This direct joining is carried out in an approximate vacuum, sealing the cavity 1104 to maintain the approximate vacuum.
Next, as illustrated in FIG. 12C, the handle layer 1109 and the buried oxide film layer 1110 are removed by etching or polishing, forming the membrane member 1102. Finally, as illustrated in FIG. 12D, the electrode 1105 is formed. Here, although FIG. 11 and FIGS. 12A to 12D illustrate only an element, a plurality of elements is generally arranged in a one-dimensional or two-dimensional array.
Unfortunately, the process for producing a capacitive electromechanical conversion device can cause a poorly bonded portion in the step of bonding together the silicon single-crystal surface and the silicone oxide surface. An element having a poorly bonded portion can fail to function as a capacitive electromechanical conversion device adequately. Poor bonding is caused partly by the accumulation (at the bonded interface) of water and/or oxygen generated at the bonded interface. Water and oxygen come from a hydroxy group (OH) involved in the direct bonding. As a method of solving this problem, a proposal where poor bonding in direct bonding is reduced by annealing is disclosed (see Arturo A. Ayon et al., Characterization of silicon wafer bonding for Power MEMS applications, Sensors and Actuators A 103 (2003) 1-8). In addition, there is a proposal of a technology relating to the arrangement of an absorbing material and an absorbing agent to absorb the gas generated at the bonded interface (see U.S. Pat. No. 6,958,255).